How to import ".off" object file using python code
You are under a misapprehension that you can not use the import OFF addon you posted.

Get the name of the operator by mousing over the import menu bpy.ops.import_mesh.off(...)

Typing the operator in the python console and using autocomplete CrtlSpace exposes the parameters (Same as those in the UI).  

bpy.ops.import_mesh.off(filepath="", filter_glob="*.off", axis_forward='Y', axis_up='Z')
&gt;&gt;&gt; bpy.ops.import_mesh.off(


Simply pass the filepath of an OFF file to the operator in your own code.

Eg: to import with default settings.

filepath = "some/path/to/file.off"
bpy.ops.import_mesh.off(filepath)


This can be done for pretty much all import / export addons.